Hydrothermally stable, conformal, sulfated zirconia monolayer catalysts for glucose conversion to 5.HMF

Article abstract of DOI:10.1021/acscatal.5b00965

The grafting and sulfation of zirconia conformal monolayers on SBA-15 to create mesoporous catalysts of tunable solid acid/base character is reported. Conformal zirconia and sulfated zirconia (SZ) materials exhibit both Br?nsted and Lewis acidity, with the Bro.nsted/Lewis acid ratio increasing with film thickness and sulfate content. Grafted zirconia films also exhibit amphoteric character, whose Bro.nsted/Lewis acid site ratio increases with sulfate loading at the expense of base sites. Bilayer ZrO₂/SBA-15 affords an ordered mesoporous material with a high acid site loading upon sulfation and excellent hydrothermal stability. Catalytic performance of SZ/SBA-15 was explored in the aqueous phase conversion of glucose to 5-HMF, delivering a 3-fold enhancement in 5-HMF productivity over nonporous SZ counterparts. The coexistence of accessible solid basic/Lewis acid and Bro.nsted acid sites in grafted SZ/SBA-15 promotes the respective isomerization of glucose to fructose and dehydration of reactively formed fructose to the desired 5-HMF platform chemical.

Full text of DOI:10.1021/acscatal.5b00965

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Article
Hydrothermally stable, conformal sulfated zirconia
monolayer catalysts for glucose conversion to 5-HMF.
Amin Osatiashtiani, Adam Fraser Lee, Marta Granollers, D. Robert
Brown, Luca Olivi, Gabriel Morales, Juan A Melero, and Karen Wilson
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00965 • Publication Date (Web): 02 Jun 2015
Downloaded from http://pubs.acs.org on June 9, 2015
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ACS Catalysis
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Hydrothermally stable, conformal sulfated zirconia monolayer
catalysts for glucose conversion to 5-HMF.
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a
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d
Amin Osatiashtiani , Adam F. Lee , Marta Granollers , D. Robert Brown , Luca Olivi , Gabriel Moꢀ
c
c
a
rales , Juan A. Melero , Karen Wilson *
a
European Bioenergy Research Institute (EBRI), Aston University, Aston Triangle, Birmingham, B4 7ET, UK
b
Department of Chemical Sciences, University of Huddersfield, Huddersfield, HD1 3DH, UK
c
Department of Chemical and Energy Tech, Chemical and Environmental Tech, Mechanical Tech and Analytical Chemistry,
Universidad Rey Juan Carlos, C/Tulipan. s/n. Eꢀ28933, Móstoles, Madrid, Spain
d
Sincrotrone Trieste, AREA Science Park, 34149 Basovizza, Trieste, Italy
ABSTRACT: The grafting and sulfation of zirconia conformal monolayers on SBAꢀ15 to create mesoporous catalysts of tunable
solid acid/base character is reported. Conformal zirconia and sulfated zirconia (SZ) materials exhibit both Brönsted and Lewis acidꢀ
ity, with the Brönsted:Lewis acid ratio increasing with film thickness and sulfate content. Grafted zirconia films also exhibit amꢀ
photeric character, whose Brönsted:Lewis acid site ratio increases with sulfate loading at the expense of base sites. Bilayer
ZrO /SBAꢀ15 affords an ordered mesoporous material with a high acid site loading upon sulfation and excellent hydrothermal staꢀ
2
bility. Catalytic performance of SZ/SBAꢀ15 was explored in the aqueous phase conversion of glucose to 5ꢀHMF, delivering a threeꢀ
fold enhancement in 5ꢀHMF productivity over nonꢀporous SZ counterparts. The coꢀexistence of accessible solid basic/Lewis acid
and Brönsted acid sites in grafted SZ/SBAꢀ15 promotes the respective isomerization of glucose to fructose and dehydration of reacꢀ
tivelyꢀformed fructose to the desired 5ꢀHMF platform chemical.
KEYWORDS: SBAꢀ15, Sulfated zirconia, Solid acid, 5ꢀHMF, Hydrothermal Stability, Solid Base
13
ity. Furthermore it would be desirable for catalysts to exhibit biꢀ
functional properties such as tuneable acid:base character to initiꢀ
1
. INTRODUCTION
1
4
ate cascade reactions in biomass conversion processes. Carbon,
15
The quest for sustainable resources to meet the demands of a rapꢀ
idly rising global population while reducing anthropogenic CO2
emissions and associated climate change represents one of this
century’s grand challenges. If average global temperature rises
induced by greenhouse gases are not to exceed 2 °C, then estiꢀ
mates indicate that a large proportion of oil, gas and coal reserves
zirconia and titania supports are amongst materials reported to
exhibit excellent hydrothermal stability and desirable characterisꢀ
tics for application in biomass processing. However, in their naꢀ
tive form they exhibit low surface areas, and offer limited control
over internal porosity, which restricts their application for the
chemical conversion of bulky bioꢀbased molecules.
1
must remain untouched. Biomass offers the most readily impleꢀ
We reported previously on the aqueous phase conversion of
glucose to 5ꢀHMF using a bifunctional sulphated zirconia (SZ)
catalyst, in which the amphoteric nature of zirconia was exploited
in conjunction with controlled surface sulfation to tune the acidꢀ
mented, and low cost solution for carbon neutral transportation
2
fuels, and the only nonꢀpetroleum route to organic molecules for
3
the manufacture of bulk, fine and speciality chemicals and polyꢀ
4
16
mers required to meet future societal demands. Akin to petroleꢀ
base properties. Tailoring the acid:base distribution can generate
um refining, biorefining will integrate biomass conversion proꢀ
cesses to produce fuels, power, and chemicals, thereby increasing
the economic viability of bioꢀderived processes. In this respect,
a material with the necessary base sites for glucose→fructose
isomerisation, and Brönsted acid sites optimized for the subseꢀ
quent conversion of reactivelyꢀformed fructose to 5ꢀHMF. Altꢀ
hough such nonꢀporous zirconias are promising, their intrinsic rate
of 5ꢀHMF productivity requires significant enhancement to imꢀ
prove catalyst efficiency and underpin a commercial process.
5
lignocellulosic biomass offers great potential for green production
6
of fuels and chemicals. Among these chemicals, 5ꢀ
7
8,9
hydroxymethylfurfural (5ꢀHMF), identified by the US DoE as
a versatile intermediate for the production of valueꢀadded chemiꢀ
cals and high performance liquid fuels, has attracted worldwide
Methods to stabilize highly porous and/or dispersed tunable ZrO
2
17
and SO /ZrO phases are thus required.
4
2
10
attention.
The direct synthesis of mesoporous ZrO and ZrO /SiO has
2 2 2
18,19,20,21
22,23
Aqueous phase, hydrothermal processing of cellulose and
sugars offers an attractive method to produce platform chemicals
been reported via sol–gel
or coꢀprecipitation methods.
However, poor thermal stability and associated poreꢀcollapse
during catalyst activation has limited the utility of such templated
11
and fuels. In a biorefinery, aqueous sugar streams may be proꢀ
duced from lignocellulose via fractionation methods such as
2
4,25
zirconias.
Robust high surface area supports such as
1
2
26
steam explosion and enzymatic hydrolysis. The development of
heterogeneous catalysts for aqueous phase conversion of such
resources requires materials with improved hydrothermal stabilꢀ
nanostructured silicas (e.g. HMSꢀ24, MCMꢀ41 and SBAꢀ15)
have the potential to serve as high area scaffolds with wellꢀ
defined mesoporosity over which to disperse ZrO , but in order to
2
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maintain the internal pore network and hence molecular accessiꢀ
electronic microscope equipped with EDX. Xꢀray Photoelectron
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bility, it is critical that zirconia is introduced in a layerꢀbyꢀlayer
fashion so as to minimise the formation of low area three dimenꢀ
sional crystallites and/or pore blockage. Incipient wetness imꢀ
Spectroscopy (XPS) was performed on a Kratos Axis HSi photoeꢀ
lectron spectrometer equipped with a charge neutralizer and Mg
K Xꢀray source (hν = 1253.6 eV). Spectra were recorded at norꢀ
α
27ꢀ32
33
pregnation,
urea hydrolysis and vapor induced hydrolysis
mal emission with an analyzer pass energy of 20 eV and Xꢀray
25,34
(VIH)
methods have been reported to prepare ZrO coatings
power of 225 W. ZrO film thicknesses were calculated from the
2
2
27ꢀ31
on SBAꢀ15, typically employing zirconium propoxide
or aceꢀ
attenuation of the Si 2p signal using equation 1:
11
tate precursors in an anhydrous organic solvent (e.g. dry hexane
or 50:50 vol% anhydrous toluene:ethanol). However, simple variꢀ
ation of the precursor concentration to achieve different zirconia
loadings invariably yields 3ꢀ4 nm crystallites, similar to the silica
mesopore diameter. VIH methods employing zirconium oxychloꢀ
)
(Equation 1)
where I is the peak intensity, I is the intensity of the clean SBAꢀ
0
0
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0
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5
15 surface, λ is the inelastic mean free path for Si 2p photoelecꢀ
trons taken to be 1.26 nm, d the film thickness and θ the angle of
ride (ZrOCl .8H O) or zirconyl nitrate precipitation with urea or
2
2
NH /water have succeeded to introduce zirconium hydroxide
37,38
3
photoelectron emission.
within SBAꢀ15 mesopores, however attempts to sulphate the reꢀ
34
Acid and base site loadings and adsorption enthalpies were
determined via adsorption flow calorimetry of NH and SO reꢀ
sulting zirconia films yield a material prone to S leaching.
3
2
Here we report a synthetic route to high area SZ/SBAꢀ15 cataꢀ
lysts possessing good textural properties and molecular access to
inꢀpore ZrO and SO active sites. The evolution of acid/base
spectively on a Setaram DSC111 system connected to gas flow
and switching systems. Samples were outgassed at 450 °C under
2
4
ꢀ1
flowing N (10 ml.min ) for 2 h prior to pulse titration at 150 °C.
2
properties was explored as a function of the thickness and degree
of sulfation of conformal zirconia monolayers (MLs), and the
resulting physicochemical properties correlated with catalytic
performance in the aqueous phase telescopic conversion of gluꢀ
cose→fructose→5ꢀHMF.
ꢀ1
A steady 10 ml.min flow of N was maintained across the samꢀ
2
ple for 3 h at 150 °C to effect activation. A sequence of 10 probe
gas pulses (1 vol% of probe gas in N ) were delivered to the carriꢀ
2
er gas stream from a 2 ml sample loop for NH /SO using a twoꢀ
3
2
position Valco valve with an automated microꢀelectric actuator.
Heat output associated with interaction between the probe gas and
the sample was detected by DSC, and the concentration of
NH /SO in the gas flow downstream of the DSC was measured
2
. EXPERIMENTAL
3
2
with a HPR 20 Hiden MS gas analyzer via a heated capillary at
75 °C. The time between pulses was 90 min for NH and 30 min
2.1 Catalyst preparation. ZrO grafted SBAꢀ15 was synthesized
2
1
3
5
3
according to our previous report. Briefly, 10 g of SBAꢀ15 (preꢀ
for SO to allow desorption of any reversibly adsorbed probe gas
3
6
2
pared via the original method of Zhao et al ) was dried at 300 °C
for 4 h, then cooled to 100 °C and added to a solution of 58.5 g of
back into the pure N stream, and/or redistribution on the sample,
2
and baseline stabilization. Temperature Programmed Desorption
was performed on probeꢀsaturated samples by ramping the temꢀ
7
0 % zirconium propoxide in propanol (SigmaꢀAldrich) in 300 ml
of anhydrous hexane. The amount of zirconium precursor was
calculated based on the number of surface hydroxyl groups on
SBAꢀ15 (determined from thermal gravimetric analysis). By
working in dry solvent it was hoped that reaction would be conꢀ
fined between surface hydroxyls and zirconium propoxide, faꢀ
vouring conformal monolayer growth over the SBAꢀ15 surface.
The reaction mixture was refluxed at 69 °C overnight, filtered and
washed three times with hexane to remove any unreacted precurꢀ
sor. The material was subsequently rehydrated in 300 ml deionꢀ
ized water under stirring for 4 h to fully hydrolyze any residual
propoxide groups. Finally, the catalyst was filtered and dried at 80
ꢀ1
perature from 150ꢀ400 °C at 5 °C.min .
Exꢀsitu pyridine adsorption was performed by exposure of
diluted samples (10 wt% in KBr) to pyridine vapour overnight.
Excess physisorbed pyridine was removed in vacuo at 30 °C prior
to recording in vacuo Diffuse Reflectance Infraꢀred Fourier Transꢀ
form (DRIFT) spectra at 50 °C in an environmental cell. Spectra
were obtained using a Nicolet Avatar 370 MCT with Smart Colꢀ
lector accessory. Zr Kꢀedge transmission Xꢀray absorption specꢀ
troscopy (XAS) was performed at the XAFS beamline of the Eletꢀ
tra synchrotron with a Si(111) doubleꢀcrystal monochromator at
2
50 mA / 2 GeV.
°C overnight. This procedure was repeated to produce SBAꢀ15
coated with a nominal second and third monolayer of zirconia.
2
.3 Catalytic tests. Kinetic studies of glucose (SigmaꢀAldrich
Sulfation of ZrO /SBAꢀ15 materials was achieved via their imꢀ
2
9
9.5%) and fructose (SigmaꢀAldrich 99%) conversion were conꢀ
mersion in aqueous sulphuric acid (10 ml acid solution per g samꢀ
ple, H SO concentrations spanning 0.005ꢀ0.25M) for 5 h, after
ducted in a Radleys Starfish carousel reactor under stirred batch
conditions at 100 °C. Reactions were performed using 0.1 g reacꢀ
tant, 0.1 g catalyst, and 20 ml deionized water. Samples were
withdrawn periodically and filtered prior to analysis on an Agilent
2
4
which samples were filtered and dried overnight at 80 °C. The
resulting materials were activated by calcination at 550 °C in
static air for 3 h.
1
200 series HPLC equipped with Refractive Index and UV diode
array detectors, and a HiꢀPlex H column. Product yields and seꢀ
lectivity were calculated on a carbon basis according to Equations
2
.2 Catalyst Characterization. N porosimetry was performed
2
on either a Micromeritics TRISTAR 3000 or Quantachrome Nova
4000 porosimeter, with BET surface areas calculated over the
range P/P0 = 0.03ꢀ0.19, wherein a linear relationship was mainꢀ
tained. Pore size distributions were obtained applying the BJH
model to the desorption branch of the isotherm. Structural order
was evaluated by means of low angle Xꢀray powder diffraction
2
and 3:
(
Equation 2)
Equation 3)
(
(XRD) on a Bruker D8 Advance diffractometer using the Cu Kα
line in the range 2θ =0.6ꢀ5.0 ° with a step size of 0.02°, with
phase identification evaluated by wide angle XRD in the range
2θ=10ꢀ80 ° with a step size of 0.04 °. HRTEM measurements
were performed on a Philips TECNAIꢀ20T electronic microscope
operated at 200 kV. Bulk zirconium contents were determined by
scanning electron microscopy (SEM) on a PHILIPS XL 30 ESEM
Mass balances were determined based on the moles of carbon in
the identified products, relative to those in the reactant according
to Equation 4:
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film morphology from the parent zirconia overlayer (e.g. sulfateꢀ
induced sintering).
1
2
3
(
Equation 4)
Initial reaction rates were determined during the first 3 h, when
conversion profiles were linear and <25 %, with resulting activiꢀ
ties reported either normalized per mass of catalyst or zirconium.
It should be noted low reaction temperatures were selected for this
study to minimise background rates for glucose and fructose conꢀ
version and facilitate the detailed study of catalyst acidꢀbase propꢀ
erties on activity.
Calculated film thickness / nm
a)
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1
1
1
1
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0
0.4
0.8
1.2
1.6
100
80
0
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0
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0
20 nm
2
1
.4 Hydrothermal stability. The hydrothermal stability of SBAꢀ
5 and ZrO /SBAꢀ15 was evaluated in a Parr 5513 autoclave. The
60
2
reactor was charged with 30 ml deionized water and 0.1 g cataꢀ
lyst, sealed and heated to 170 °C under vigorous stirring for 6 h.
Materials were held at this temperature at ~7 bar autogenous presꢀ
sure, then cooled to room temperature, recovered by filtration and
then dried at 80 °C overnight prior to analysis of textural properꢀ
ties by porosimetry and XRD.
40
2
0 nm
20
0
20 nm
3
. RESULTS AND DISCUSSION
3
.1 ZrO and SZ film growth. The successful stepwise growth of
0
1
2
3
2
zirconia monolayers over SBAꢀ15 was first verified by a combiꢀ
nation of porosimetry, HRTEM, XRD and XPS. Nitrogen poꢀ
rosimetry revealed that all materials exhibited type IV isotherms
and steep H1 hysteresis loops typical of mesostructured SBAꢀ15
with a narrow mesopore size distribution (Fig. S3 and S4). The
composition and textural properties of the parent SBAꢀ15 and
Number of grafting cycles
b)
39
ZrO grafted samples are reported in Table S1 and S2 as a funcꢀ
tion of nominal monolayers of ZrO , and are in accordance with
2
2
1
ML
2 ML
our recent communication.ref22 Pore volume, mean mesopore
diameter and BET surface area all decreased progressively with
each grafting cycle, consistent with a layerꢀbyꢀlayer growth mode
of zirconia throughout the mesopore network, resulting in thickenꢀ
ing of the mesopore walls and a concomitant reduction of the
mesopore diameter. Low angle XRD (Fig S1) also confirmed the
mesostructured order of SBAꢀ15 was retained, with materials
exhibiting a main peak at 2θ = ~1° and two weaker features at 2θ
Fig 1. (a) Attenuation of Si 2p XP signal of ZrO /SBAꢀ15 as a
2
function of zirconia grafting cycle (dashed line shows the theoretꢀ
ical attenuation predicted from equation 1), inset shows HRTEM
of pore channels following 2 and 3 grafting cycles; and (b) Estiꢀ
mated film thickness for 1 and 2 ML ZrO over a SiO substrate.
2
2
=
(
port.
1.7° and 1.9° attributed respectively to the (100), (110) and
200) reflections of the pmm6 hexagonally ordered parent supꢀ
The bulk and surface S:Zr atomic ratios fell with initial ZrO2
thickness, indicating only the terminating zirconia layer underꢀ
went sulfation (Table S2).
3
6,40
The highly dispersed nature of the incorporated ZrO2
was verified by the absence of monoclinic or tetragonal phase
diffraction features in the wide angle XRD region, indicating
dimensions below the instrumental detection limit of around 2
nm. These observations are supported by HRTEM, which visualꢀ
ized an ordered hexagonal SBAꢀ15 mesophase with uniform pore
walls following Zr grafting, with no evidence for crystalline zirꢀ
conia deposits even after three grafting cycles (Fig 1a). Layerꢀbyꢀ
layer growth of zirconia overlayers was verified by XPS via atꢀ
tenuation of the Si 2p signal; Fig 1a reveals an exponential decay
in the SBAꢀ15 substrate intensity with consecutive zirconia depoꢀ
sitions. The first and second grafting cycles attenuated the subꢀ
strate by 35.6 and 51.9 %, equating to 0.5 and 0.84 nm thick overꢀ
Acid site strength, loading and nature was probed by a combiꢀ
nation of NH calorimetry and pyridine titration. Fig 2 shows NH
3
3
pulse calorimetry profiles for SZ/SBAꢀ15, which evidence preꢀ
dominantly moderate strength acidic sites (ꢀꢀH ~100ꢀ120
ads
16
ꢀ
1
kJ.mol ), comparable to those in bulk SZ catalysts. The 1 and 2
ML SZ/SBAꢀ15 samples also exhibit a small proportion of strong
ꢀ1
acid sites with ꢀꢀHads ~180ꢀ200 kJ.mol . While the precise origin
of these strong acid sites is unclear, they likely originate through
the truncated nature of the ZrO monolayers, and corresponding
2
4+
defective (uncoordinated Zr ) centres which may confer strong
Lewis acidity, being electronically perturbed by the underlying
41
silica. This hypothesis is supported by pyridine titration and
4
2
layers of ZrO respectively, in excellent agreement with the estiꢀ
mated thicknesses of (111) oriented monoꢀ and bilayers of ZrO2
2
quantification of the resultant DRIFT spectra (Fig 3 and S7)
which show a switchover from Lewis to Brönsted acidity with
(
Fig 1b), confirming their conformal nature.
Sulfation of the 1ꢀ3 ML ZrO /SBAꢀ15 samples was subseꢀ
quently performed by impregnation with 0.075 M H SO (the
increasing ZrO film thickness. The Zr 3d XP spectra (Fig S5)
2
2
also exhibits a shift to lower binding energy with increased layer
thickness, demonstrating the electron deficient nature of the first
2
4
concentration previously reported to afford optimum acid site
layer at the SiO interface. The maximum acid site density (Fig 3)
2
1
6
properties) and calcination at 550 °C. The absence of any new
XRD feastures postꢀsulfation confirmed the absence of large SZ
crystallites. Furthermore, the Zr:Si atomic ratios from XPS and
EDX were unchanged by sulfation, suggesting no change in the
was following sulfation of a zirconia bilayer. Interfacial zirconia
thus appears either more chemically inert due to perturbation by
the underlying SBAꢀ15 support, or an inability to crystallize in the
4
3
requisite tetragonal phase known to impart superacidity. The
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acid:base density was independent of zirconia film thickness,
Tetragonal ZrO comprises two nonꢀequivalent tetrahedral ZrO
2 4
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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3
3
3
3
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6
being dominated by solid acid character.
units, with different ZrꢀO bond lengths. Enhanced sꢀp mixing in
45,46
tetrahedral geometries
gives rise to an extra electronic state
200
~6ꢀ7 eV below the Zr Kꢀedge white line which is unique to teꢀ
tragonal zirconia, and consistent with peak A, evidencing the
1
2
MLꢀSZ/SBAꢀ15
MLꢀSZ/SBAꢀ15
presence of tetragonal ZrO in both ZrO /SBAꢀ15 and SZ/SBAꢀ15
180
160
140
120
100
80
60
2
2
materials. Sulfation causes the white line to sharpen, with the
lower energy feature B strengthening at the expense of C, indicatꢀ
ing a change in electronic structure. The rise in B is consistent
with the changes in symmetry and charge distribution of the Zr
local environment expected to accompany their covalent binding
3MLꢀSZ/SBAꢀ15
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
to SO groups. Fig 4 inset shows the radial distribution functions
4
of ZrO and SZ films, revealing features at 2.0 and 3.3 Å, attributꢀ
2
able to ZrꢀO and ZrꢀZr first coordination shells; the weak ZrꢀZr
shell consistent with the ultraꢀthin nature of the conformal film
which hence lacks the complete nearest neighbour shell present
within threeꢀdimensional zirconia crystallites.
0
0.1
0.2
0.3
0.4
0.5
B
ꢀ
1
Surface coverage / mmol g
C
Fig 2. NH pulse calorimetry of 1ꢀ3 ML SZ/SBAꢀ15.
3
The highly dispersed nature of the conformal ZrO monolayꢀ
2
ers prohibited their phase analysis by conventional powder XRD,
hence Zr Kꢀedge XAS was performed to inform the structure of
both zirconia and SZ films. The common cubic, tetragonal and
monoclinic ZrO phases exhibit characteristic XANES spectra,
2
A
SO ꢀ2ML ZrO /SBA
4 2
whose lineshape and width are dictated by symmetryꢀdependent
2
ML ZrO /SBA
2
44,45
splitting of the 1s→4d transition.
Symmetrical cubic ZrO (8ꢀ
2
4+
coordinate Zr ) affords a sharp edge jump and two well resolved
transitions to e and t valence states. In contrast, tetragonal (8ꢀ
coordinate Zr ) and monoclinic (7ꢀcoordinate Zr ) phases posꢀ
sess lower symmetry, leading to further splitting of the Zr 4d vaꢀ
lence band.
g
2g
4
+
4+
B
0.45
1.4
A
0
2
4
6
1
1
0
0
.2
Bond length / Å
C
0
.3
.8
.6
ꢀ20 ꢀ10
0
10 20 30 40
Energy above edge / eV
0
.15
0
0.4
Fig 4. Normalized and derivative Zr Kꢀedge XANES of 2 ML
0
0
.2
ZrO /SBAꢀ15 and 2 ML SZ/SBAꢀ15. Inset shows corresponding
2
radial distribution functions from the EXAFS spectra.
1
ML
2 ML
3 ML
SZ coverage / ML
3
.2 Catalytic activity. The catalytic performance of 1ꢀ3 ML
SZ/SBAꢀ15 in glucose dehydration to 5ꢀHMF at 100 C is sumꢀ
marized in Fig 5. Activity for glucose conversion and resultant 5ꢀ
HMF production increases from 1 2 ML, showing little further
°
Fig 3. Acid and base properties of 1ꢀ3 ML SZ/SBAꢀ15.
→
Tetragonal ZrO Kꢀedge XANES split into four (a , b , e and b )
change for 3 ML. Glucose conversion to 5ꢀHMF is proposed to
proceed via a Lewis acid (or base) catalyzed isomerization to
fructose, followed by a Brönsted acid catalyzed dehydration as
shown in Scheme 1. The catalytic performance mirrors both the
total acid site loading and Brönsted:Lewis acid ratio, suggesting
that 5ꢀHMF production is rateꢀlimited by the Brönsted acid cataꢀ
lysed dehydration. This postulate is supported by the observed
accumulation of fructose during reaction (Fig. S20), which reꢀ
2
1
1
2
bands which are typically resolved as a doublet, while the monoꢀ
clinic phase possesses five nonꢀresolvable states observed as a
featureless single peak at the white line. Fig 4 shows that the 2
ML ZrO /SBAꢀ15 exhibits three distinct features, a weak preꢀedge
2
feature labelled A, and a split white line labelled B and C, which
are more clearly discerned in the derivative spectra.
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ACS Catalysis
flects its slower removal through dehydration relative to producꢀ
tion via more rapid glucose isomerisation.
≤0.05 M, however for [H SO ] ≥0.1 M both S and Zr loadings
2 4
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
decreased, suggesting corrosion and/or dissolution of the ZrO2
film. This hypothesis was confirmed by the Zr:Si and S:Si surface
atomic ratios, which both fell under aggressive sulfation treatꢀ
ment. SZ films prepared with [H SO ] <0.1 M, with S:Zr <0.18
0.25
12
2
4
therefore appear chemically stable. NH and SO pulse calorimeꢀ
3
2
try mapped the evolution of acid and base site loadings with S:Zr
atomic ratio for the 2 ML SZ/SBAꢀ15 (Fig S15 and S16 and Ta-
ble S7); acid sites increased steadily with S content at the expense
of base sites, in excellent agreement with previous trends obꢀ
1
8
6
4
2
0
0
0
.2
0
.15
16
served for bulk SZ. At S:Zr ratios >0.18 the acid site loading
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
reached a plateau, indicating surface saturation with sulfate speꢀ
ꢀ
1
cies. The mean ꢀ ꢁHads (NH ) of 110 kJ.mol was independent of
3
0
.1
sulfate coverage, indicating a common, predominantly medium
16
strength acid site, consistent with bulk SZ, while the mean ꢀ
ꢀ
1
ꢁ
Hads(SO ) of 100 kJ.mol for 2 ML ZrO /SBAꢀ15 was conꢀ
2
2
0.05
0
sistent with values for bulk ZrO
which possesses weak/medium
2
ꢀ1
base sites; typical heats of adsorption for SO span 60 kJ.mol
2
ꢀ
1
(
for SiO ) to 215 kJ.mol (MgO), hence values for 2 ML
2
ꢀ1
47
SZ/SBAꢀ15 materials of 80ꢀ129 kJ.mol evince mild basicity.
0
1
2
3
4
The evolution of Brönsted:Lewis acid character with θSO4 was
quantified by pyridine titration, and confirmed the expected inꢀ
SZ coverage / ML
crease in Brönsted acid character with sulfation (Fig S18)
.
Fig 5. Comparison of activity for glucose conversion and HMF
production at as a function of SZ film thickness.
3
.4 Catalytic activity. The performance of xMꢀSZ/SBAꢀ15 cataꢀ
lysts was subsequently evaluated for 5ꢀHMF production from
fructose or glucose. 5ꢀHMF productivity per Zr atom from frucꢀ
tose was directly proportional to the concentration of sulphating
solution (i.e. θSO4) (Fig S21), while in contrast that from glucose
exhibited a maximum for [H SO ] = 0.02 M, equivalent to an
Solid Base
Brönsted
or
Acid
Lewis Acid
2
2
3 H O
3 H O
2
4
acid:base atomic ratio of approximately 4. These observations are
qualitatively consistent with our recent study of fructose converꢀ
sion over bulk SZ catalysts, and in quantitative agreement with
that for glucose conversion wherein maximal 5ꢀHMF productivity
was attained for a bulk SZ surface with an identical mix of acꢀ
id:base character. The rise in 5ꢀHMF production from fructose
with surface sulfation can be simply understood in terms of the
concomitant increase in Brönsted acidity and hence dehydration
activity, whereas glucose conversion to 5ꢀHMF requires a balance
of surface base/Lewis acid sites (to drive isomerization to frucꢀ
tose) and Brönsted acid sites (for the subsequent dehydration).
Despite similarities in the catalytic behaviour of xMꢀSZ /SBAꢀ15
and bulk SZ materials, the high area conformal SZ monolayers
delivered far superior performance in respect of 5ꢀHMF producꢀ
tivity per Zr atom, being three (two) times more active for glucose
Scheme 1. Glucose isomerization to fructose and subsequent deꢀ
hydration to 5ꢀHMF
.
3
.3 Effect of sulfate coverage. Our previous studies of bulk SZ
demonstrated that the telescopic glucose fructose 5ꢀHMF reacꢀ
→
→
tion could be tuned through controlling the degree of surface sulꢀ
fation of the underlying amphoteric zirconia, i.e. SO coverage
4
1
6
(
θ
). We hence investigated the impact of θSO4 on the behaviour
of the optimal 2 ML ZrO /SBAꢀ15 material via impregnation with
2
0
.005ꢀ0.25 M H SO . Low angle XRD and N porosimetry conꢀ
2 4 2
firmed retention of the parent SBAꢀ15 structure for all acid molarꢀ
ities (Fig S9 and S11), although samples treated with >0.1 M
H SO induced a slight expansion in the mean mesopore diameter
2
4
may indicate a small degree of zirconia reꢀdispersion (Table 1).
XPS and EDX (Fig S13 and S14 and Table S5 and S6) confirmed
that the Zr content was constant with rising S loading for [H SO ]
(fructose) conversion than bulk SZ analogues (Fig 6).
2
4
Table 1. Structure, composition and acid/base properties of 2 ML SZ/SBA-15 as a function of H SO molarity
2
4
Sample
Atomic S:Zr Surface area
Mesopore
diameter
Bulk Zr
(EDX)
Bulk S
(EDX)
Acid site
loading
Base site
loading
2
ꢀ1
(XPS)
/ m .g
/
wt%
/ wt%
ꢀ1
ꢀ1
/
nm
6.8
5.4
5.6
5.6
5.3
5.3
4.3
5.9
6.3
/ mmol.g
/ mmol.g
SBAꢀ15
ꢀ
850
540
550
538
506
547
542
557
483
0
0
2 ML ZrO /SBAꢀ15
ꢀ
18.7
20.7
19.9
20.0
18.8
16.5
11.8
9.5
0.0
0.0
0.0
0.6
1.3
1.5
1.5
1.7
0.233
0.079
2
0.005M SZ/SBAꢀ15
0.01
0.02
0.06
0.13
0.18
0.22
0.42
0
.01M SZ/SBAꢀ15
0.275
0.302
0.34
0.067
0.039
0.017
0.008
0
.025M SZ/SBAꢀ15
0
.05M SZ/SBAꢀ15
0.1M SZ/SBAꢀ15
0.397
0
.17M SZ/SBAꢀ15
.25M SZ/SBAꢀ15
0
0.347
0.002
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2
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40
20
00
Bulk SZ
SZ/SBA
recovered Zr/SBA-15
Fresh Zr/SBA-15
recovered SBA-15
Fresh SBA-15
8
0
0
6
0
1
2
3
4
5
6
7
8
9
0
1
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8
9
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0
1
2
3
4
5
6
7
8
9
0
40
1
2 3 4 5 6
2
θ
2
0
Bare SBAꢀ15
0
Glucose
Fructose
Reactant
Fig 6. Zr normalized 5ꢀHMF productivity from glucose and frucꢀ
tose over the optimum bulk SZ and SZ/SBAꢀ15 catalysts. (SZ and
SZ/SBA prepared from impregnation with 0.02M H SO or 0.1 M
2
ML Zr/SBAꢀ15
2
4
H SO for glucose or fructose reactions respectively).
3
6
9
12 15 18 21
2
4
Diameter / nm
This difference may reflect the greater dispersion of the thin film
system, and increased Lewis character of the grafted SZ film
compared to bulk SZ, and hence improved glucose isomerisation
of the former. In this respect, we note that while Lewis acids are
Fig 7. BJH pore size distributions before (filled symbols) and
after (open symbols) hydrothermal treatment of parent SBAꢀ15
(
ꢀ
) and 2 ML ZrO /SBAꢀ15 (
ꢀ
). Inset shows low angle XRD of
asꢀprepared and postꢀhydrothermally treated materials.
claimed to be more selective than strong bases like Ca(OH) or
2
2
4
8
Mg(OH) for glucose→fructose isomerisation (which promote
2
4
9
undesired degradation via reverse aldol reactions ), in this work
the mild basicity of ZrO seems important in promoting 5ꢀHMF
4. CONCLUSIONS
2
formation at low surface θSO4.The recyclability of our SZ/SBAꢀ15
materials in glucose conversion to 5ꢀHMF was evaluated for two
of the most active catalysts, which were recovered after 6 h reacꢀ
tion, dried and reꢀcalcined. Recycled catalysts retained their activꢀ
ity for glucose dehydration, confirming their robust nature (Fig
S22-23).
A conformal grafting method has been employed to grow ZrO2
monolayers over a mesoporous SBAꢀ15 framework. XAS, XPS
and DRIFTS analysis confirms that subsequent wet impregnation
with H SO results in highly dispersed sulphated tetragonal zircoꢀ
2
4
nia monolayers whose acid site density and Brönsted:Lewis acid
site ratio is optimal for a 2 ML SZ film. The acid:base properties
of these conformal SZ films can be readily tuned by varying the
concentration of the sulphating solution; high [H SO ] favouring
3.5 Hydrothermal stability. Heterogeneous catalysts that possess
excellent hydrothermal stability are essential to the development
of aqueous phase biomass processing. While the reaction temꢀ
peratures used in this work were mild, related biomass processes
2
4
Brönsted surface acidity. These 2 ML SZ/SBAꢀ15 catalysts were
examined towards the aqueous phase conversion of glucose to 5ꢀ
HMF, displaying significant per site rate enhancements for 5ꢀ
HMF production than bulk SZ counterparts under identical reacꢀ
tion conditions. Coꢀexisting basic and Lewis acid sites (associated
with zirconia exposed within partially sulphated conformal ZrO2
films) in SZ/SBAꢀ15 materials promote glucose isomerization to
fructose, while Brönsted acid (sulfate) sites direct fructose dehyꢀ
dration to 5ꢀHMF. While the absolute performance of our sulfated
ZrꢀSBAꢀ15 catalysts is not exceptional compared to other proꢀ
cesses reported, this reflects our interest in developing a green
and sustainable process employing water as the solvent, which is
known to afford poorer HMF yields than organic solvents such as
DMSO (which facilitate sugar dehydration but is not a practical
solvent for large scale application). This rationale for selecting an
aqueous solvent is simple; all current biomass preꢀtreatment
routes to sugars, e.g. steam explosion, or enzymatic/chemical
(acid or base) cellulose hydrolysis, produce aqueous sugar
operate between 140ꢀ180
°C, a regime wherein ordered mesopoꢀ
rous silicas such as SBAꢀ15 exhibit poor hydrothermal
3
6,50ꢀ52
53
stability,
hindering their wider utility. Since bulk ZrO is
2
considered hydrothermally stable, we speculated that dispersion
of zirconia over SBAꢀ15 could improve the support stability in
high temperature water. Fig 7 shows pore size distributions and
low angle XRD patterns for asꢀsynthesised and 2 ML ZrO /SBAꢀ
15 before and after heating in water at 170 °C for 6 h.
The parent SBAꢀ15 underwent pore collapse, evidenced by loss of
low angle diffraction features and simultaneous broadening of the
pore distribution. The zirconia grafted SBAꢀ15 by contrast reꢀ
tained an ordered pore network with uniform, narrow mesopores.
Hence it can be concluded that grafting zirconia is beneficial to
hydrothermal stability of the SBAꢀ15 support. Dissolution of
2
10
54ꢀ56
amorphous silica has been reported by many research groups
,
5
7
even at room temperature. While silica dissolution from SBAꢀ15
leads to loss of order and pore expansion, the zirconia film proꢀ
tects the SBAꢀ15 support and the structure of the support remains
intact.
12
streams, hence the development of heterogeneous catalysts able
to operate in water is essential. It is also important to note that our
reaction temperature of 100
literature reports (>140 C) in order to minimize unwanted side
reactions and humin formation, which is problematic at high temꢀ
perature and often poorly quantified The finding that zirconia
°C is significantly lower than most
°
.
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ACS Catalysis
monolayers confer excellent hydrothermal stability to SBAꢀ15
Vries, J. G. Chemical Reviews 2013, 113, 1499ꢀ
597.
11) Peterson, A. A.; Vogel, F.; Lachance, R.
P.; Froling, M.; Antal, J. M. J.; Tester, J. W.
Energy & Environmental Science 2008, 1, 32ꢀ65.
(12) Giang, C.; Osatiashtiani, A.; dos Santos,
V.; Lee, A.; Wilson, D.; Waldron, K.; Wilson, K.
Catalysts 2014, 4, 414ꢀ426.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
suggests that ZrO /SBAꢀ15 is a promising support material for
2
1
more widespread application to aqueous phase reforming.
(
■
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
Low and wide angle XRD patterns, tables with structural parameꢀ
ters and bulk and surface compositions, N porosimetry isotherms
2
and pore size distribution figures, XP spectra, DRIFT spectra for
pyridine titration, SO and NH titration and calorimetry figures,
(13) Stöcker, M. Angewandte Chemie
International Edition 2008, 47, 9200ꢀ9211.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
3
tables of catalytic tests for glucose and fructose dehydration to 5ꢀ
HMF corresponding reaction profiles, normalized activities and
(14) Elmekawy, A. A.; Shiju, N. R.;
recyclability tests, N porosimetry isotherms and a table of physiꢀ
2
cal properties before and after hydrothermal stability test.
Rothenberg, G.; Brown, D. R. Industrial &
Engineering Chemistry Research 2014, 53,
■
AUTHOR INFORMATION
1
8722ꢀ18728.
Corresponding Author
*Eꢀmail: k.wilson@aston.ac.uk.
Notes
(15) SerranoꢀRuiz, J. C.; Luque, R.;
SepulvedaꢀEscribano, A. Chemical Society
Reviews 2011, 40, 5266ꢀ5281.
The authors declare no competing financial interest.
(16) Osatiashtiani, A.; Lee, A. F.; Brown, D.
5
. ACKNOWLEDGEMENTS
R.; Melero, J. A.; Morales, G.; Wilson, K.
Catalysis Science & Technology 2014, 4, 333ꢀ
342.
Financial support from the European Union Seventh Framework
Programme (FP7/2007ꢀ2013) under grant agreement no. 604307
is gratefully acknowledged. In addition we are also grateful to the
EPSRC for funding under EP/K014676/1 and for the provision of
Leadership Fellowship to AFL (EP/G007594/4). KW is grateful
to the Royal Society for the award of an Industry Fellowship
(17) Grecea, M. L.; Dimian, A. C.; Tanase,
S.; Subbiah, V.; Rothenberg, G. Catalysis Science
& Technology 2012, 2, 1500ꢀ1506.
(
IF100206). JAM and GM gratefully acknowledge the "Ministerio
de Ciencia e Innovación" of Spain through the project CTQ2011ꢀ
8216, and the Regional Government of Madrid through the proꢀ
(18) Fang, H.; Wan, T.; Shi, W.; Zhang, M.
2
Journal of NonꢀCrystalline Solids 2007, 353,
1657ꢀ1661.
ject S2013ꢀMAEꢀ2882. Dr Mark Isaacs is also acknowledged for
his assistance with XPS measurements.
(
19) Wong, M. S.; Huang, H. C.; Ying, J. Y.
Chemistry of Materials 2002, 14, 1961ꢀ1973.
20) Wu, Z.ꢀG.; Zhao, Y.ꢀX.; Liu, D.ꢀS.
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